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Abstract:

A superimposed signal detection circuit detects a signal superimposed on
an optical signal in a WDM system. The superimposed signal detection
circuit includes: an optical filter having wavelength-dependent loss to
filter a plurality of optical signals on which a corresponding
superimposed signal is superimposed by frequency modulation; a photo
detector to convert the plurality of optical signals filtered by the
optical filter into an electric signal; and a detector to obtain
information indicated by the superimposed signal respectively
superimposed on the plurality of optical signals from the electric signal
obtained by the photo detector.

Claims:

1. A superimposed signal detection circuit that detects a signal
superimposed on an optical signal in a WDM system, the superimposed
signal detection circuit comprising: an optical filter having
wavelength-dependent loss to filter a plurality of optical signals on
which a corresponding superimposed signal is superimposed by frequency
modulation; a photo detector to convert the plurality of optical signals
filtered by the optical filter into an electric signal; and a detector to
obtain information indicated by the superimposed signal respectively
superimposed on the plurality of optical signals from the electric signal
obtained by the photo detector.

2. The superimposed signal detection circuit according to claim 1,
wherein a relation between a free spectral range of the optical filter
and a spacing of wavelength channels of the WDM system is expressed by a
ratio of integers.

3. The superimposed signal detection circuit according to claim 1,
wherein a relation between a free spectral range of the optical filter
and a resolution of a frequency slot width of the WDM system is expressed
by a ratio of integers.

4. The superimposed signal detection circuit according to claim 1,
wherein the optical filter includes: a plurality of optical filter
elements; an optical demultiplexer to separate a WDM optical signal with
respect to wavelength to be guided to the plurality of the optical filter
elements, the WDM optical signal including a plurality of optical signals
on which corresponding superimposed signal is respectively superimposed
by frequency modulation; a controller to control a transmittance of the
plurality of optical filter elements; and an optical combiner to combine
output optical signals of the plurality of optical filter elements.

5. The superimposed signal detection circuit according to claim 4,
wherein the controller controls a transmittance of an optical filter
element corresponding to a specified wavelength channel in the WDM
optical signal to change depending on wavelength, and the detector
detects a superimposed signal superimposed on an optical signal of the
specified wavelength channel.

6. The superimposed signal detection circuit according to claim 1,
wherein a bandwidth of the photo detector is lower than a symbol rate of
data signal carried by the optical signal, and higher than half of a
modulation rate of the superimposed signal.

7. Optical node equipment provided in a WDM transmission system, the
optical node equipment comprising: an optical filter having
wavelength-dependent loss to filter a plurality of optical signals on
which a corresponding superimposed signal is superimposed by frequency
modulation; a photo detector to convert the plurality of optical signals
filtered by the optical filter into an electric signal; and a detector to
obtain information indicated by the superimposed signal respectively
superimposed on the plurality of optical signals from the electric signal
obtained by the photo detector.

8. The optical node equipment according to claim 7, wherein the optical
filter is a wavelength blocker, a wavelength selective switch, a
wavelength selective demultiplexer, or a wavelength selective
multiplexer.

9. The optical node equipment according to claim 8, wherein the photo
detector is an optical channel monitor provided in the wavelength
blocker, the wavelength selective switch, the wavelength selective
demultiplexer, or the wavelength selective multiplexer.

10. The optical node equipment according to claim 7, wherein the optical
filter includes: a plurality of optical filter elements; an optical
demultiplexer to separate a WDM optical signal with respect to wavelength
to be guided to the plurality of the optical filter elements, the WDM
optical signal including a plurality of optical signals on which
corresponding superimposed signal is respectively superimposed by
frequency modulation; a controller to control a transmittance of the
plurality of optical filter elements; and an optical combiner to combine
output optical signals of the plurality of optical filter elements.

11. The optical node equipment according to claim 10, wherein the
controller controls a transmittance of an optical filter element
corresponding to a wavelength channel to which a selection instruction
indicating a first state being applied so as to block input light; and
the controller controls transmittance of an optical filter element
corresponding to a wavelength channel to which a selection instruction
indicating a second state being applied so as to change depending on a
wavelength.

12. The optical node equipment according to claim 11, wherein the
controller controls a transmittance of an optical filter element
corresponding to a wavelength channel to which a selection instruction
indicating a third state being applied so as to pass input light
substantially without depending on a wavelength.

13. An optical transmission system including a WDM transmission equipment
that transmits a WDM optical signal and optical node equipment that
processes the WDM optical signal; wherein the WDM transmission equipment
includes an optical transmitter to superimpose a corresponding
superimposed signal on a plurality of optical signals included in the WDM
optical signal; and the optical node equipment includes: an optical
filter having wavelength-dependent loss to filter the plurality of
optical signals; a photo detector to convert the plurality of optical
signals filtered by the optical filter into an electric signal; and a
detector to obtain information indicated by the superimposed signal
respectively superimposed on the plurality of optical signals from the
electric signal obtained by the photo detector.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is based upon and claims the benefit of priority
of the prior Japanese Patent Application No. 2011-141670, filed on Jun.
27, 2011, the entire contents of which are incorporated herein by
reference.

FIELD

[0002] The embodiments described herein are related to a superimposed
signal detection circuit to detect a signal superimposed on an optical
signal, and an optical node equipment having a function to detect a
superimposed signal.

BACKGROUND

[0003] A photonic network having an optical add-drop multiplexer and/or a
wavelength crossconnect has been proposed and developed. The optical
add-drop multiplexer (ROADM: Reconfigurable Optical Add/Drop Multiplexer)
is capable of dropping an optical signal of a desired wavelength from a
WDM optical signal and guiding the dropped signal to a client, and
capable of adding a client signal of any wavelength to a WDM optical
signal. The wavelength crossconnect (WXC: Wavelength Cross Connect or
PXC: Photonic Cross Connect) is capable of controlling the route of an
optical signal for each wavelength, without converting the optical signal
into an electric signal.

[0004] In a photonic network as described above, a plurality of optical
paths (here, wavelength paths) that use the same wavelength may be set.
For this reason, in order to establish and operate a network certainly,
for example, a scheme to superimpose a path ID to identify each optical
path on an optical signal has been proposed. In this case, optical node
equipment (here, the optical add-drop multiplexer, the wavelength
crossconnect and the like) has a function to detect the path ID
superimposed on the optical signal. Accordingly, since each optical path
can be identified certainly at the optical node equipment, it becomes
possible to monitor/detect/avoid a failure such as to connect the optical
fiber to a wrong port, and so on.

[0005] As a technique to manage the optical path, a method having the
following steps has been proposed. The steps includes combining at least
one payload data stream with at least one side data stream comprising the
path ID into a composite electrical data stream; applying the composite
data stream to an optical transmitter to produce an optical signal;
detecting the optical signal with an optical receiver having a maximum
frequency of operation less than one-half of the rate of the composite
data stream; and recovering the side data stream from the electrical
output of the optical receiver. (for example, U.S. Pat. No. 7,580,632).

[0007] In a conventional art (for example, FIG. 2 and FIG. 3a in U.S. Pat.
No. 7,580,632 and the like), the signal representing the path ID
(hereinafter, a path ID signal) is superimposed on the optical signal by,
for example, intensity modulation. In this case, cross gain modulation
occurs by an optical amplifier that amplifies the WDM optical signal
collectively (for example, EDFA) and/or by induced Raman scattering in
the optical fiber. The cross gain modulation may induce crosstalk of the
path ID signal between wavelength channels in the WDM optical signal. As
a result the path ID may be identified wrongly in the optical node
equipment.

[0008] In another conventional art, after modulating a data signal using a
code corresponding to the path ID in the electric domain, an optical
signal is generated by optical modulation by the modulated data signal.
In this case, for the optical receiver, an optical demodulator
corresponding to the optical modulation scheme needs to be provided on
the input side of a converter to convert the optical signal into the
electric signal. Therefore, in a system in which a plurality of optical
modulation schemes are used, a plurality of optical modulators need to be
provided, increasing the circuit size. In addition, when the payload data
have different symbol rates, it is difficult to collectively adjust the
clocks of respective wavelength channels.

SUMMARY

[0009] According to an aspect of the invention, a superimposed signal
detection circuit that detects a signal superimposed on an optical signal
in a WDM system. The superimposed signal detection circuit includes: an
optical filter having wavelength-dependent loss to filter a plurality of
optical signals on which a corresponding superimposed signal is
superimposed by frequency modulation; a photo detector to convert the
plurality of optical signals filtered by the optical filter into an
electric signal; and a detector to obtain information indicated by the
superimposed signal respectively superimposed on the plurality of optical
signals from the electric signal obtained by the photo detector.

[0010] The object and advantages of the invention will be realized and
attained by means of the elements and combinations particularly pointed
out in the claims.

[0011] It is to be understood that both the foregoing general description
and the following detailed description are exemplary and explanatory and
are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

[0012] FIG. 1 is a diagram illustrating an example of an optical
transmission system in which a superimposed signal detection circuit of
an embodiment is used.

[0013] FIG. 2 is a diagram illustrating the configuration of WDM
transmission equipment.

[0014] FIG. 3A and FIG. 3B are diagrams illustrating the configuration of
an optical transmitter having a function to superimpose a path ID signal.

[0031] Each of the WDM transmission equipments 2-5 transmits and receives
a WDM optical signal including a plurality of optical signals of
different wavelengths. Each of the optical add-drop multiplexers 6-8
drops an optical signal of a specified wavelength from an input WDM
optical signal and guides the dropped signal to a client line. In
addition, each of the optical add-drop multiplexers 6-8 adds an optical
signal input from a client line to a WDM optical signal. The wavelength
crossconnect 9 has a plurality of input ports and a plurality of output
ports, and guides an input signal to a corresponding output port so as to
realize a specified optical path. While it is not explicitly illustrated,
the wavelength crossconnect 9 may also have a function for dropping a
signal from a WDM optical signal to a client circuit and for adding a
client signal to a WDM optical signal.

[0032] The network management system 10 sets an optical path specified by
a user in the optical transmission system 1. In other words, the network
management system 10 controls the WDM transmission equipments 2-5, the
optical add-drop multiplexers 6-8 and the wavelength crossconnect 9 so as
to realize an optical path specified by the user. The network management
system 10 may be in a configuration to establish a so-called management
plane by a centralized control system, or may be in a configuration to
establish a so-called control plane by a distributed control system, or,
may be a system being a combination of them.

[0033] In the example illustrated in FIG. 1, optical paths P1-P4 are set
in the optical transmission system 1. Each optical path is indicated by a
broken line. The optical path 1 carries an optical signal from the WDM
transmission equipment 2 to the WDM transmission equipment 4 via the
optical add-drop multiplexer 6 and the wavelength crossconnect 9. The
optical path 2 carries an optical signal from the WDM transmission
equipment 2 to a client 11 via the optical add-drop multiplexer 6. The
optical path 3 carries an optical signal from the WDM transmission
equipment 3 to a client 12 via the optical add-drop multiplexer 7. The
optical path P4 carries an optical signal from a client 13 to the WDM
transmission equipment 5 via the optical add-drop multiplexer 7, the
wavelength crossconnect 9 and the optical add-drop multiplexer 8. Each of
the optical paths P1-P4 may carry optical signal in both directions.

[0034] In the optical transmission system 1, the network management system
10 may assign the same wavelength to different optical paths, to utilize
the communication resource efficiently or flexibly. In the example
illustrated in FIG. 1, wavelengths λ1, λ3, λ1, λ1
are assigned to the optical paths P1, P2, P3, P4, respectively.

[0035] The user or the network administrator may wish to check whether the
optical paths are set correctly. However, when the same wavelength is
assigned to a plurality of optical paths, it is difficult to identify
each optical path only by monitoring the spectrum of each wavelength
channel. For example, at the wavelength crossconnect 9, it is difficult
to identify the optical paths P1, P4 only by monitoring the spectrum of
each wavelength.

[0036] Therefore, the network management system 10 assigns a path ID to
each optical path. The transmitting source equipment of an optical path
superimposes a path ID signal that represents the path ID on an optical
signal to be transmitted. For example, the WDM transmission equipment 2
superimposes a path ID signal that represents "path ID=1" on an optical
signal to be transmitted via the optical path 1, and superimposes a path
ID signal representing "path ID=2" on an optical signal to be transmitted
via the optical path 2.

[0037] The optical node equipment has a superimposed signal detection
circuit 14 to detect the path ID signal superimposed on the optical
signal and to obtain the path ID. The optical node equipment corresponds
to the optical add-drop multiplexer 6-8 and the wavelength crossconnect
9. However, the superimposed signal detection circuit 14 does not have to
be provided for all of the optical node equipments, and a plurality of
superimposed signal detection circuits 14 may be provided for one optical
node equipment. In addition, the superimposed signal detection circuit 14
may be built within the optical node equipment, or may be connected to
the optical node equipment. The superimposed signal detection circuit 14
may also be provided for the WDM transmission equipment 2-5.

[0039] Each of the optical transmitters 21-1 through 21-n generates an
optical signal by modulating carrier light by input data sequence. Here,
wavelengths λ1 through λn (that is, optical frequencies) of
the carrier light that the optical transmitters 21-1 through 21-n use are
different from each other. To the optical transmitters 21-1 through 21-n
that transmit the optical signal, a path ID is assigned by the network
management system 10. The path ID is given to the corresponding optical
transmitters 21-1 through 21-n as a path ID signal. The path ID signal
is, for example, a code of a specified length. In this case, the codes to
identify the respective optical paths are orthogonal to each other. In
addition, the path ID signals may be tone signals of different
frequencies from each other. The tone signal is, for example, a sine wave
signal, while there is no particular limitation. The rate of the path ID
signal (the bit rate of the code, the frequency of the tone signal) is
sufficiently low compared with the rate of the data sequence.

[0040] The optical transmitters 21-1 through 21-n superimpose the path ID
signal on the optical signal by frequency modulation. In other words, the
optical transmitters 21-1 through 21-n outputs an optical signal on which
the path ID signal is superimposed by frequency modulation. The
multiplexer 22 multiplexes optical signals output from the optical
transmitters 21-1 through 21-n to generate a WDM optical signal.

[0041] The modulation scheme of the data sequence by the optical
transmitters 21-1 through 21-n does not have to be the same as each
other. For example, the optical transmitter 21-1 may transmit a QPSK
modulated optical signal, while the optical transmitter 21-1 may transmit
a 16QAM modulated optical signal. In addition, the symbol rate or the bit
rate of optical signals output from the optical transmitters 21-1 through
21-n does not have to be the same as each other.

[0042] The demultiplexer 24 demultiplexes a received WDM optical signal to
output optical signals to the optical receivers 23-1 through 23-n. The
optical receivers 23-1 through 23-n respectively demodulate the optical
signals to recover data sequence transmitted from a corresponding
transmitter.

[0043] FIG. 3A and FIG. 3B illustrate the configuration of the optical
transmitter having a function to superimpose the path ID signal. The
optical transmitter illustrated in FIG. 3A and FIG. 3B is an example of
the optical transmitters 21-1 through 21-n illustrated in FIG. 2.
However, the configuration to superimpose the path ID signal on an
optical signal by frequency modulation is not limited to the
configuration or the method illustrated in FIG. 3A and FIG. 3B.

[0044] The optical transmitter illustrated in FIG. 3A has a frequency
tunable laser light source 31 and an optical modulator 32. The frequency
tunable laser light source 31 generates continuous wave light of an
oscillation frequency based on a frequency control signal. Therefore, by
giving the path ID signal as a frequency control signal, the frequency
tunable laser light source 31 can generate continuous wave light of an
oscillation frequency according to the path ID signal. The optical
modulator 32 modulates the continuous wave light generated by the
frequency tunable laser light source 31 by the data sequence. As a
result, an optical signal on which the path ID signal is superimposed by
frequency modulation is generated.

[0045] The optical transmitter illustrated in FIG. 3B realizes the
frequency modulation superimposition by a digital signal processing. A
mapping circuit 33 maps the data sequence into an I component data
sequence and a Q component data sequence. An integrator circuit 34
integrates the path ID signal. In the configuration illustrated in FIG.
3B, the path ID signal f(t) is a digital data sequence representing an
amplitude-time waveform of a code or a tone signal. The integrator
circuit 34 outputs phase information θ(t) described below as the
result of the integration.

θ(t)=∫2πf(t)dt

A mod 2π circuit 35 converts the output value of the integrator
circuit 34 into a value within a range from zero to 2π. However, when
the value range of the integrator circuit 34 is designed to be from zero
to 2π, the mod 2π circuit 35 may be omitted.

[0046] A rotation operation circuit 36 rotates the I component data
sequence and the Q component data sequence using the phase information
θ(t) by the operation below, where I, Q are input data of the
rotation operation circuit 36 and I', Q' are output data of the rotation
operation circuit 36.

I'=I cos θ(t)-Q sin θ(t)

Q'=I sin θ(t)+Q cos θ(t)

[0047] The data I' and the data Q' obtained by the rotation operation
circuit 36 are respectively converted into an analog signal by a D/A
converter 37 and given to the optical modulator 38. The optical modulator
38 generates a modulated optical signal by modulating the continuous wave
light output from the laser light source 39 by the data I' and the data
Q'. As a result, an optical signal on which the path ID signal is
superimposed by frequency modulation is generated. When the limitation of
frequency bandwidth, the frequency characteristic ripple, nonlinear
response, delay difference (skew) are not negligible as the analog
characteristic of the D/A converter 37, the optical modulator 38 and the
connection line between them, they may be corrected by providing a
digital signal processor having opposite characteristics to them at the
input side of the D/A converter 37.

[0048] FIG. 4 is a diagram explaining the frequency modulation
superimposition. FIG. 4 illustrates the time resolved output spectrum of
the optical transmitter at time T0, T1-T4. The spread of the output
spectrum at each time represents the spectrum spread generated according
to the high speed modulation by the data sequence, and may take various
widths and forms according to the modulation scheme and the modulation
speed of the optical signal. On the optical signal output from the
optical transmitter, as explained with reference to FIG. 3A and FIG. 3B,
the path ID signal is superimposed by frequency modulation. In the
example illustrated in FIG. 4, the path ID signal is a digital code, and
it is assumed that the path ID code superimposed on the optical signal at
the time T1-T4 is "0110". In addition, it is assumed that the center
frequency of carrier light used by the optical transmitter is f1.

[0049] At the time T0, the path ID code is not superimposed on the optical
signal. In this case, the optical transmitter does not shift the
frequency of the optical signal. Therefore, the center of the spectrum of
the optical signal output at the time T0 is f1.

[0050] At the time T1, "0" is superimposed on the optical signal. In this
embodiment, the optical transmitter shifts the frequency of the optical
signal by -Δf. Therefore, the center of the spectrum of the optical
signal output at the time T1 is f1-Δf.

[0051] At the time T2, "1" is superimposed on the optical signal. In this
case, in this embodiment, the optical transmitter shifts the frequency of
the optical signal by +Δf. Therefore, the center of the spectrum of
the optical signal output at the time T2 is f1+Δf. In the same
manner, the center of the spectrum of the optical signal output at the T3
is f1+Δf, and the center of the spectrum of the optical signal
output at the time T4 is f1-Δf.

[0052] The frequency shift Δf is sufficiently small compared with
the frequency of the carrier light. In addition, Δf is determined
to avoid ICI (inter channel interference) in the WDM transmission system.
For example, in a WDM transmission system in which the wavelength channel
is arranged on the 50 GHz/100 GHz frequency grid recommended by ITU-T,
appropriate Δf is about 1 MHz-1 GHz, while this is not a particular
limitation. When Δf is too small, the frequency fluctuation (laser
line width) of the carrier light becomes normegligible as noise, and the
detection sensitivity of the superimposed signal at the superimposed
signal detection circuit becomes low. Therefore, it is preferable to
determine Δf in consideration of the interference with adjacent
channels and the detection sensitivity.

[0053] While the frequency shift when the superimposed signal is "0" and
"1" is "-Δf" and "+Δf" respectively in the example
illustrated in FIG. 4, the present embodiment is not limited to this
scheme. For example, the frequency shift when the superimposed signal is
"0" and "1" may be "+Δf" and "-Δf" respectively. In addition,
the frequency shift may be zero when the superimposed signal is "0 (or,
1)", and the optical frequency may be shifted when the superimposed
signal is "1 (or, 0)". Furthermore, as 4-level frequency shift keying,
for example, the frequency shift when two bits of the superimposed signal
is "00", "01", "10" and "11" may be "-Δf", "-0.5Δf",
"+0.5Δf" and "+Δf", respectively. Furthermore, the
superimposed signal may be modulated using multi-level frequency shift
keying other than binary or 4-level.

[0054] While the path ID signal is a digital signal in the example
illustrated in FIG. 4, the method to shift the optical frequency is
substantially the same in the case in which the path ID signal is an
analog signal. However, when the path ID signal is an analog signal, the
amount of frequency shift changes continuously (not discretely).

[0055] FIG. 5 illustrates the configuration of a superimposed signal
detection circuit. A superimposed signal detection circuit 40 of the
embodiment has an optical filter 41, a low-speed photo detector 42, and a
detector 43. The superimposed signal detection circuit 40 corresponds to
the superimposed signal detection circuit 14 provided in the optical node
equipment, in the optical transmission system illustrated in FIG. 1.

[0056] The optical filter 41 filters an input optical signal. The
transmittance (or, loss) of the optical filter 41 depends on the
wavelength (or optical frequency) of the optical signal. That is, the
optical filter 41 is a wavelength-dependent loss optical filter. In other
word, the optical filter 41 is a wavelength-dependent transmittance
optical filter.

[0057] The low-speed photo detector 42 includes a photo diode for example,
and outputs an optical current corresponding to the intensity of the
input optical signal. That is, the low-speed photo detector 42 converts
the optical signal filtered by the optical filter 41 into an electric
signal. The bandwidth of the optical detector 42 is assumed to be lower
than the symbol rate of the data signal carried by the optical signal.
While there is no particular limitation, for example, the bandwidth of
the low-speed photo detector 42 may be lower than or equal to 1% of the
symbol rate of the data signal carried by the signal. Therefore, the data
signal is averaged at the low-speed photo detector 42. However, the
bandwidth of the low-speed photo detector 42 is supposed to be
sufficiently high with respect to the purpose of realizing the
demodulation of the path ID signal superimposed on the optical signal.
While there is no particular limitation, for example, the bandwidth of
the low-speed photo detector 42 may be lower than or equal to 1 percent
of the symbol rate of the data signal carried by the signal and higher
than or equal to 0.5 times the frequency modulation rate of the path ID
signal.

[0058] The detector 43 detects the path ID signal from the electric signal
obtained by the low-speed photo detector 42, and obtains the path ID.
When a WDM optical signal including a plurality of optical signals is
input to the superimposed signal detection circuit 40, the detector 43
detects path ID signals respectively superimposed on the optical signals,
and obtains each path ID. That is, the detector 43 is capable of
identifying the path ID superimposed on the optical signal at the optical
transmitter. The detector 43 may detect the path ID signals respectively
superimposed on a plurality of optical signals collectively and
simultaneously. The detector 43 may be configured to perform
presence/absence judgment as to whether or not an expected path ID has
been superimposed on the optical signal, and to output the
presence/absence judgment result.

[0059] FIG. 6 is a diagram illustrating the operation of the optical
filter 41 and the low-speed photo detector 42. FIG. 6 includes a diagram
illustrating the spectrum of the optical signal, a diagram illustrating
the characteristics of the optical filter 41, and a diagram illustrating
the average power of the output light of the optical filter 41.

[0060] The spectrum 0 is the spectrum of the optical signal when the
superimposed signal is "0", and the center frequency of the spectrum 0 is
f1-Δf. The spectrum 1 is the spectrum of the optical signal when
the superimposed signal is "1", and the center frequency of the spectrum
1 is f1+Δf. The spectrum presented by the broken line indicates the
state where no superimposed signal is given, and the center frequency is
f1.

[0061] The transmittance of the optical filter 41 depends on the
wavelength (or optical frequency) of the optical signal as described
above. In the example illustrated in FIG. 6, the optical filter 41 is
designed so that the transmittance increases as the optical frequency
becomes higher, and the transmittance decreases as the optical frequency
becomes lower, in the frequency area around f1.

[0062] Here, it is assumed that when an optical signal on which the path
ID signal is not superimposed (that is, the optical signal of the center
frequency f1) is input to the optical filter 41, the average output
optical power of the optical filter 41 is P1. In this case, when the
optical signal on which "0" is superimposed (that is, the optical signal
whose center frequency is f1-Δf) is input to the optical filter 41,
since the transmittance decreases due to the frequency shift, the average
output optical power of the optical filter 41 is smaller than P1. On the
other hand, when the optical signal on which "1" is superimposed (that
is, the optical signal whose center frequency is f1+Δf) is input to
the optical filter 41, since the transmittance increases due to the
frequency shift, the average output optical power of the optical filter
41 is larger than P1.

[0063] Averaging of the output optical power of the optical filter 41 is
realized by the low-speed photo detector 42. That is, the low-speed photo
detector 42 generates an electric signal that represents the average
optical power of the optical signal filtered by the optical filter 41.
For example, when the path ID code "0110" illustrated in FIG. 4 is
superimposed on the optical signal, the low-speed photo detector 42
sequentially outputs "0" "1" "1" "0", where "0" corresponds to the state
where the optical power represented by the output signal of the low-speed
photo detector 42 is smaller than P1, and "1" corresponds to the state
where the optical power represented by the output signal of the low-speed
photo detector 42 is larger than P1. Thus, the optical filter 41 and the
low-speed photo detector 42 are capable of converting the path ID
superimposed on the optical signal by frequency modulation at the optical
transmitter into an intensity modulated signal.

[0064] FIG. 7A and FIG. 7B illustrate examples of the detector 43. The
detector 43 detects the path ID signal from the electric signal obtained
by the low-speed photo detector 42 and obtains the path ID. Here, the
superimposed signal detection circuit 40 detects the path ID signals
superimposed respectively on a plurality of optical signals included in a
WDM optical signal.

[0065] FIG. 7A illustrates an example of the detector 43 in a case in
which the path ID identifying the optical path is realized by a code. The
bit length of the path ID codes representing the path IDs are supposed to
be the same as each other. In addition, the WDM optical signal can
accommodate n wavelength channels. In this case, the detector 43 has a
sampling unit 51, shift registers 52-1 through 52-m, correlators 53-1
through 53-m, decision circuits 54-1 through 54-m. m is any integer that
is equal to or larger than 1, and n and m may be equal to each other.

[0066] The sampling unit 51 samples the electric signal output from the
low-speed photo detector 42. The frequency of the sampling clock may be
the same as the bit rate (or the chip rate) of the path ID code, for
example. The sampled data sequence obtained by the sampling unit 51 is
guided to the shift registers 52-1 through 52-m. The length of the shift
registers 52-1 through 52-m is the same as the bit length of the path ID
code.

[0067] To the correlators 53-1 through 53-m, corresponding codes 1 through
m are given, respectively, The codes 1 through m are given from the
network management system 10 illustrated in FIG. 1, for example. In
addition, the codes 1 through m are codes expected to be superimposed on
the optical signals in the input WDM optical signal. The correlators 53-1
through 53-m respectively calculates the correlation of the codes 1
through m and the sampled data sequence held in the shift registers 52-1
through 52-m.

[0068] The decision circuits 54-1 through 54-m respectively compare the
correlation value calculated by the correlators 53-1 through 53-m and a
threshold. Then, the decision circuits 54-1 through 54-m judge whether or
not the codes 1 through m are detected, based on the result of the
comparison. For example, when the correlation value calculated by the
correlator 53-1 is higher than the threshold, the decision circuit 54-1
decides that the code 1 is detected from the input WDM optical signal. In
this case, the superimposed signal detection circuit 40 decides that the
optical path identified by the code 1 has been established in the input
WDM optical signal. On the other hand, when the correlation value
calculated by the correlator 53-1 is equal to or lower than the
threshold, the decision circuit 54-1 decides that the code 1 is not
detected from the input WDM optical signal. In this case, the
superimposed signal detection circuit 40 decides that the optical path
identified by the code 1 has not been established in the input WDM
optical signal.

[0069] The correlators 53-1 through 53-m are capable of calculating the
correlation of the corresponding codes 1 through m and the sampled data
sequence independently from each other and in parallel. In addition, the
decision circuits 54-1 through 54-mare capable of comparing the
correlation values calculated by the correlators 53-1 through 53-m and
the threshold independently from each other and in parallel. Therefore,
the superimposed signal detection circuit 40 can decide whether or not
the codes 1 through m are detected from the input WDM optical signal
simultaneously and collectively. That is, the superimposed signal
detection circuit 40 is capable of deciding whether or not the optical
paths identified by the codes 1 through mare established in the input WDM
optical signal simultaneously and collectively.

[0070] FIG. 7B illustrates an example of the case in which the path ID
identifying the optical path is realized by a tone signal. The frequency
of tone signals 1 through m representing the path ID is different from
each other. Here, it is supposed that, in the WDM transmission equipment
illustrated in FIG. 2, the frequency of the tone signals 1 through m
superimposed on each optical signal is λ(t1) through λ(tm),
respectively. In addition, the WDM optical signal can accommodate n
wavelength channels. In this case, the detector 43 has bandpass filters
55-1 through 55-m and decision circuits 56-1 through 56-m.

[0071] In the detector 43, the electric signal output from the low-speed
photo detector 42 is fed to the bandpass filters 55-1 through 55-m. The
passing frequency of the bandpass filters 55-1 through 55-m are
λ(t1) through λ(tm), respectively.

[0072] The decision circuits 56-1 through 56-m respectively compare the
output level of the bandpass filters 55-1 through 55-m with a threshold.
The decision circuits 56-1 through 56-m judge whether or not the tone
signals 1 through m are detected, based on the result of the comparison.
For example, when the output level of the bandpass filter 55-1 is higher
than the threshold, the decision circuit 56-1 decides that the tone
signal 1 is detected from the input WDM optical signal. In this case, the
superimposed signal detection circuit 40 decides that the optical path
identified by the tone signal 1 has been established in the input WDM
optical signal. On the other hand, when the output level of the bandpass
filter 55-1 is equal to or lower than the threshold, the decision circuit
56-1 decides that the tone signal 1 is not detected from the input WDM
optical signal. In this case, the superimposed signal detection circuit
40 decides that the optical path identified by the tone signal 1 has not
been established in the input WDM optical signal.

[0073] The bandpass filters 55-1 through 55-m are capable of filtering the
electric signal output from the low-speed photo detector 42 independently
from each other and in parallel. The bandpass filters 55-1 through 55-m
may be configured so that the center frequency of their passing band is
variable. In addition, the decision circuits 56-1 through 56-m are
capable of comparing the output levels of the bandpass filters 55-1
through 55-m with the threshold independently from each other and in
parallel. Therefore, superimposed signal detection circuit 40 is capable
of deciding whether or not the tone signals 1 through m are detected from
the input WDM optical signal collectively and simultaneously. That is,
the superimposed signal detection circuit 40 is capable of deciding
whether or not the optical paths identified by the tone signals 1 through
m are established in the input WDM optical signal collectively and
simultaneously. In addition, the bandpass filters 55-1 through 55-m and
the decision circuit 56-1 through 56-m may be configured collectively by
a signal processing circuit including an A/D converter, signal buffer
memory, and a high-speed Fourier transform circuit.

[0074] Thus, the superimposed signal detection circuit of the embodiment
is capable of detecting the path ID signal superimposed on an optical
signal, without depending on the modulation scheme to carry the data
signal. Therefore, for the optical node equipment, there is no need to
provide an optical modulation circuit corresponding to the optical
modulation scheme to detect the path ID signal from the optical signal.
Accordingly, the superimposed signal detection circuit is realized by a
simple configuration, and the size becomes small.

[0075] Here, the parallel number m for the decision circuits 54-1 through
54-m or 56-1 through 56-m in the detector 43 may be equal to the maximum
number of wavelength channels n that the WDM optical signal can
accommodate. According to such a design, it becomes possible to minimize
the time required for the decision to confirm that the path ID is
detected as expected. Meanwhile, m may be smaller than n. In this case,
while the circuit size of the detector 43 may be reduced, in order to
confirm that the path ID is detected as expected for all the wavelength
channels, decision process is necessary for a plurality of times while
sequentially changing the codes input to the correlator 53-1 through 53-m
or changing the center frequency of the passing band of the bandpass
filters 55-1 through 55-m, increasing the time required for the decision.
In addition, when m is larger than n, while the circuit size of the
detector 43 increases, not only the time require for decision to confirm
that the path ID is detected as expected may be minimized, but also
comparison with a path ID that is not expected to reach the detector may
be performed collectively, making it possible to detect the situation of
occurrence of a failure due to a connection mistake in the network and
the like in detail and in a short time.

[0076] FIG. 8 illustrates an example of the optical filter 41. In this
example, wavelength channels of the WDM transmission are allocated on a
frequency grid of a specified spacing. In the example illustrated in FIG.
8, the wavelength channels are set using optical frequencies f1, f2, f3,
. . . . In this case, the minimum spacing of the wavelength channels of
the optical transmission system 1 corresponds to the spacing of the
frequency grid. For example, when a frequency grid of the 50 GHz spacing
is adopted for the optical transmission system 1, the minimum spacing of
the wavelength channels of the WDM transmission is also 50 GHz.
Meanwhile, in ITU-T Recommendations G.694.1, a frequency grid including
50 GHz spacing and 100 GHz spacing is defined.

[0077] FIG. 8 illustrates the spectrum of the optical signal of each
wavelength channel. The shape of the spectrum depends on the modulation
scheme of the optical signal and the symbol rate of the optical signal.
For example, when the symbol rate becomes higher, the width of the
spectrum becomes broader. Thus, in the example illustrated in FIG. 8,
optical signals of various modulation schemes and/or symbol rates are
present.

[0078] The optical filter 41 has a characteristic that the transmittance
(or, loss) changes periodically with respect to the frequency. At this
time, the optical filter 41 is designed to have a free spectral range
(FSR) that has a relationship of an integral ratio with respect to the
minimum spacing of the wavelength channels of the WDM transmission. That
is to say, a relation between a free spectral range of the optical filter
41 and a spacing of wavelength channels of the WDM system may be
expressed by a ratio of integers. Preferably, the free spectral range of
the optical filter 41 is designed to be a fraction of an integer with
respect to the minimum spacing of the wavelength channels of the WDM
transmission. In the example illustrated in FIG. 8, the free spectral
range of the optical filter 41 is designed to be approximately the same
as the minimum spacing of the wavelength channels of the WDM
transmission.

[0079] In addition, while this is not necessarily optimal, as another
example, the free spectral range of the optical filter 41 may be designed
to have a value that is not a fraction of an integer with respect to the
minimum spacing of the wavelength channels of the WDM transmission but a
value that realizes an integral ratio with respect to each other. For
example, the free spectral range of the optical filter 41 may be twice
the minimum spacing of the wavelength channels of the WDM transmission.
In this case, since the sign of the slope of the transmittance of the
optical 41 with respect to the frequency grid of even numbers and odd
numbers become opposite, it needs to be noted that the polarity of the
optical current waveform output from the low-speed photo detector 42 are
inverted.

[0080] The transmittance of the optical filter 41 periodically repeats
peak and bottom (local minimum) with respect to the frequency. At this
time, the optical filter 41 is designed so that the frequency grid is
located in the area in which the transmittance changes from peak to
bottom or in the area in which the transmittance changes from bottom to
peak. Preferably, the optical filter 41 is designed so that the frequency
grid is located at the midpoint between peak and bottom of the
transmittance or its vicinity. By designing the transmittance of the
optical filter in this way, the frequency shift of the optical signal is
converted into the change of the optical intensity efficiently.

[0081] The optical filter whose transmittance periodically changes with
respect to the optical frequency is realized by using Fabry-Perot Etalon,
for example. With Fabry-Perot Etalon, by designing the thickness and
material of etalon appropriately, the period of the change of the
transmittance with respect to the optical frequency may be determined
arbitrarily, and a desired transmitting center wavelength may be
obtained.

[0082] The optical filter whose transmittance periodically changes with
respect to the optical frequency is not limited to Fabry-Perot Etalon.
The optical filter may be realized by an asymmetric Mach-Zehnder
interferometer (DLI: Delay Line Interferometer), FBG: Fiber Bragg
Grating, superstructure FBG, and so on.

[0083] FIG. 9 illustrates another embodiment of the optical filter 41.
Wavelength channels of the WDM transmission are allocated using a
flexible frequency grid.

[0084] In the flexible frequency grid, frequency slots are specified. Each
wavelength channel of the WDM transmission is allocated using one or a
plurality of frequency slots. For example, one frequency slot is assigned
to a wavelength channel whose symbol rate is low. A plurality of
frequency slots are assigned to a wavelength channel whose symbol rate is
high. In the example illustrated in FIG. 9, a wavelength channel to which
one frequency slot is assigned, a wavelength channel to which two
frequency slots are assigned, and a wavelength channel to which four
frequency slots are assigned are present. The width of a frequency slot
may be referred to as resolution or frequency resolution.

[0085] When the flexible frequency grid is adopted for the optical
transmission system 1, the optical filter 41 is designed so as to have a
free spectral range that has a relationship of an integral ratio with
respect to the resolution of the width of the frequency slot. That is to
say, a relation between a free spectral range of the optical filter 41
and a resolution of a frequency slot width of the WDM system may be
expressed by a ratio of integers. Preferably, the free spectral range of
the optical filter 41 is designed so as to be a fraction of an integer
with respect to the resolution of the width of the frequency slot. In the
example illustrated in FIG. 9, the free spectral range of the optical
filter 41 is designed to be approximately equal to the resolution of the
width of the frequency slot.

[0086] When the flexible frequency grid is adopted for the optical
transmission system 1, the optical filter 41 may also be realized by
using, for example, Fabry-Perot Etalon. In addition, the optical filter
41 may also be realized by DLI, superstructure FBG and the like.

[0087] FIG. 10 illustrates an example of a ROADM having a superimposed
signal detection circuit. A ROADM 60 illustrated in FIG. 10 is an example
of optical node equipment. The ROADM 60 illustrated in FIG. 10 has an
optical amplifier 61, an optical splitter 62, a wavelength blocker 63, an
optical splitter 64, an optical coupler 65, an optical amplifier 66, a
low-speed photo detector 42, and a detector 43.

[0088] The optical amplifier 61 amplifies an input WDM optical signal. The
optical splitter 62 guides the WDM optical signal amplified by the
optical amplifier 61 to the wavelength blocker 63, and also splits the
WDM optical signal to generate a drop signal. The drop signal is guided
to, for example a wavelength selective demultiplexer or a demultiplexer
that are not illustrated in FIG. 10. The wavelength selective
demultiplexer selects a specified wavelength from the drop signal and
guides the selected signal to a client terminal. The demultiplexer
separates the drop signal for each wavelength. In this case, some or all
of a plurality of optical signals obtained by the demultiplexer are
guided to the client terminals.

[0089] The wavelength blocker 63 passes a specified wavelength in the
input WDM optical signal according to an instruction from the network
management system 10, and blocks other wavelengths, for example. The
wavelength blocker 63 works as the optical filter 41 of the superimposed
signal detection circuit 40 illustrated in FIG. 5, while this to be
explained later.

[0090] The optical splitter 64 splits an optical signal output from the
wavelength blocker 63 and to be guided to the optical coupler 65 and the
low-speed photo detector 42. The optical coupler 65 couples an add signal
and the optical signal output from the optical splitter 64 to generate an
output WDM optical signal. The optical amplifier 66 amplifies the WDM
optical signal obtained by the optical coupler 65.

[0091] The operations of the low-speed photo detector 42 and the detector
43 are as explained with reference to FIG. 5. The low-speed photo
detector 42 converts the optical signal guided from the optical splitter
64 into an electric signal. The detector 43 detects a path ID
respectively superimposed on one or a plurality of optical signal in the
input WDM optical signal, from the electric signal generated by the
low-speed photo detector 42.

[0092] FIG. 11 is a diagram illustrating the optical filter realized by
the wavelength blocker 63. Here, it is assumed that the input WDM optical
signal includes wavelength channels ch1 through ch 7. The wavelength
blocker 63 provides an optical filter function according to a selection
instruction given from a user or the network management system 10, for
example. Here, the selection instruction may specify one of three states
for each wavelength channel. The first state is for blocking the input
optical signal. The second state is for detecting the path ID signal
superimposed on the input optical signal and passing the input optical
signal. The third state is for passing the input optical signal without
detecting the path ID signal. In the example illustrated in FIG. 11, the
wavelength blocker 63 has received the following selection instructions.

[0093] In this case, the wavelength blocker 63 provides the transmission
characteristics illustrated in FIG. 11. The wavelength blocker 63
provides a sufficiently small transmittance for the frequency area
corresponding to the wavelength channel ch3. In addition, the wavelength
blocker 63 provides a sufficiently large transmittance for the frequency
area corresponding to the wavelength channels ch2, ch5, and ch7. The
transmittance provided for the wavelength channels ch2, ch5, and ch7 are
approximately constant with respect to the frequency. Furthermore, the
wavelength blocker 63 provides a transmittance that is dependent on the
frequency respectively to the frequency area corresponding to the
wavelength channels ch1, ch4, and ch6. That is, the wavelength blocker 63
provides an optical filter having a frequency-dependent loss respectively
to the wavelength channels ch1, ch4, and ch6. In the example illustrated
in FIG. 11, in each frequency area corresponding to the wavelength
channels ch1, ch4, ch6, the transmittance becomes larger as a frequency
becomes high. However, depending on the features of the
configuration/characteristics and the operating status of the optical
filter, in some or all of the respective frequency areas corresponding to
ch1, ch4, ch6, the transmittance may be smaller as the frequency becomes
high.

[0094] The WDM optical signal including the wavelength channels ch1
through ch7 is input to the wavelength blocker 63. By so doing, the
optical signal of the wavelength channel ch3 is blocked by the wavelength
blocker 63. That is, the wavelength blocker 63 does not output the
optical signal of the wavelength channel ch3. In addition, the optical
signals of the wavelength channels ch2, ch5, ch7 are not blocked by the
wavelength blocker 63. That is, the wavelength blocker 63 outputs the
optical signals of the wavelength channels ch2, ch5, ch7.

[0095] The optical signals of the wavelength channels ch1, ch4, ch6 are
output after filtered by the optical filter having frequency-dependent
loss. The filtering operation using the optical filter having
frequency-dependent loss is as described with reference to FIG. 6.
Therefore, when the path ID signals are respectively superimposed by
frequency modulation on the optical signals of the wavelength channels
ch1, ch4, ch6, the average optical power of the output light of the
wavelength blocker 63 changes according to the superimposed path ID
signals. For example, when the path ID code 1 is superimposed on the
optical signal of wavelength channel ch1 and the path ID code 4 is
superimposed on the optical signal of wavelength channel ch4, the average
optical power of the output light of the wavelength blocker 63 changes
according to the path ID code 1 and the path ID code 4.

[0096] The output light of the wavelength blocker 63 is split by the
optical splitter 64 and guided to the low-speed photo detector 42. The
low-speed photo detector 42 averages the output optical power of the
wavelength blocker 63, as illustrated with reference to FIG. 6. As a
result, when the path ID signal is superimposed by frequency modulation
on the optical signals of the wavelength channels ch1, ch4, ch6, an
intensity modulated signal representing the path ID may be obtained by
the low-speed photo detector 42. For example, when the path ID code 1 and
the path ID code 4 are superimposed on the optical signals of the
wavelength channels ch1 and ch4 respectively, the intensity modulated
signal representing the path ID code 1 and the path ID code 4 is obtained
by the low-speed photo detector 42. Therefore, the detector 43 can detect
the two path IDs from the intensity modulated signal.

[0097] Thus, in the example illustrated in FIG. 10, the superimposed
signal detection circuit is realized by using the wavelength blocker 63
built within the ROADM. In this configuration, the ROADM may detect the
path ID signal superimposed on the optical signal of the specified
wavelength channel while blocking other specified wavelengths. In
addition, according to this configuration, since the wavelength blocker
63 is used as the wavelength-dependent loss optical filter, the space
efficiency in the ROADM is high.

[0098] While the optical signals of the wavelength channels ch1, ch4, ch6
are filtered by the optical filter having frequency-dependent loss, they
are output from the ROADM node without being blocked. Thus, the
superimposed signal detection circuit of the embodiment can detect path
ID signal from a desired wavelength channel while minimizing the
influence to the signal quality of the wavelength channel, by
appropriately setting the frequency-dependent loss characteristics of the
optical filter.

[0099] FIG. 12 illustrates an example of the configuration of the
wavelength blocker 63. The wavelength blocker 63 has, in this example,
demultiplex element 71, liquid crystal elements 72-1 through 72-p, and a
combiner 73. The demultiplex element 71 demultiplexes an input WDM
optical signal with respect to wavelength to be guided to the liquid
crystal elements 72-1 through 72-p.

[0100] Control terminals 74-1 through 74-p are provided for the liquid
crystal elements 72-1 through 72-p, respectively. The transmittance of
the liquid crystal elements 72-1 through 72-p is controlled by the
voltage respectively applied to the corresponding control terminals 74-1
through 74-p. The liquid crystal elements 72-1 through 72-p do not have
to be separate from each other. The combiner 73 combines the output light
of the liquid crystal elements 72-1 through 72-p.

[0101] FIG. 13 is a diagram illustrating the operation of the wavelength
blocker 63. The wavelength blocker 63 has a plurality of liquid crystal
elements as illustrated in FIG. 12, in this example. The transmittance of
each liquid crystal element depends on the voltage applied through a
corresponding control terminal. In the example illustrated in FIG. 13,
when a voltage V1 is applied, the transmittance of the liquid crystal
element is controlled to T1, where T1 is a transmittance that
substantially blocks the input light. When a voltage V2 is applied, the
transmittance of the liquid crystal element is controlled to T2, where T2
is a transmittance that passes the input signal with a small loss.

[0102] In the example illustrated in FIG. 13, three liquid crystal
elements are assigned to one wavelength channel. For example, liquid
crystal elements 72-1 through 72-3 are assigned to the wavelength channel
ch1, liquid crystal elements 72-4 through 72-6 are assigned to the
wavelength channel ch2, and liquid crystal element 72-7 through 72-9 are
assigned to the wavelength channel ch3.

[0103] Applied voltages to the liquid crystal elements 72-1 through 72-3
are different from each other. In this example, the applied voltage of
the liquid crystal element 72-2 is larger than the applied voltage of the
liquid crystal element 72-1, and the applied voltage of the liquid
crystal element 72-3 is higher than that of the liquid crystal elements
72-1, 72-2. In this case, the transmittance of the liquid crystal element
72-1, 72-2, 72-3 becomes larger in this order. As a result, an optical
filter having a sloped transmittance with respect to the frequency is
provided for wavelength channel ch1. Note that applied voltages to the
liquid crystal element 72-1 through 72-3 are close to V2, so that the
optical signal of the wavelength channel ch1 passes the wavelength
blocker 63.

[0104] To all of the liquid crystal element 72-4 through 72-6, the voltage
V2 is applied. As a result, since an optical filter having an
approximately constant transmittance T2 is provided for the wavelength
channel ch2, the optical signal of the wavelength channel ch2 passes the
wavelength blocker 63. To the liquid crystal elements 72-7 through 72-9,
the voltage V1 is applied. As a result, since an optical filter having an
approximately constant transmittance T1 is provided for the wavelength
channel ch3, the optical signal of the wavelength channel ch3 is blocked
by the wavelength blocker 63. Explanation for other wavelength channels
is omitted.

[0105] Thus, by controlling the applied voltage of each liquid crystal
element, the optical filter having a desired transmission characteristic
may be realized for each wavelength channel. Therefore, the ROADM may
detect the path ID signal superimposed on the optical signal of one or
plurality of desired wavelength channels, while blocking other specified
wavelengths in the WDM optical signal.

[0106] FIG. 14 is a diagram illustrating the control system to control the
wavelength blocker 63. In FIG. 14, a wavelength blocker controller 81
controls a control voltage applying circuit 82 according to a given
selection instruction. At this time, the wavelength blocker controller 81
may control the control voltage applying circuit 82 according to the
detection result of the path ID by the detector 43. The selection
instruction may specify the wavelength channel to be blocked, the
wavelength channel to be passed, the wavelength channel to be the target
of detecting the path ID signal. In addition, the selection instruction
is given from the user or the network management system 10, for example.
Then, the control voltage applying circuit 82 generates a voltage to be
applied to corresponding liquid crystal element in the wavelength blocker
63, according to the control by the wavelength blocker controller 81. The
wavelength blocker controller 81 may be realized by the microcomputer
including a processor.

[0107] Note that the liquid crystal elements 72-1 through 72-p in FIG. 12
may be replace by another configuration, such as optical spatial
modulator pixels that may realize spatial optical modulation such as
polarization control. For example, the liquid crystal elements 72-1
through 72-p may be realized by movable micro mirror array by MEMS (Micro
Electro Mechanical Systems) and variable hologram elements.

[0109] The wavelength selective switch 67 selects one or more specified
wavelengths from the input WDM optical signal and add signals. The
wavelength selective switch 67 provides an optical filter function
similar to the wavelength blocker 63 illustrated in FIG. 10. However, the
wavelength selective switch 67 has a plurality of input ports. That is,
the wavelength selective switch 67 has one or a plurality of ports to
receive add signals, in addition to the port to receive the input WDM
optical signal. In addition, the wavelength selective switch 67 may also
have a plurality of output ports. The configuration and the operation of
the wavelength selective 67 may be understood by those who are skilled in
the art in the technical field of the present invention from the
configuration and operation of the wavelength blocker 63 illustrated in
FIG. 10, therefore, further explanation for the wavelength selective 67
is omitted.

[0110] Thus, the superimposed signal detection circuit 40 of the
embodiment may be realized, in the ROADM 60, using the wavelength
selective switch 67, the low-speed photo detector 42, and the detector
43.

[0111] The superimposed signal detection circuit 40 may be realized by a
wavelength selective demultiplexer 91, the low-speed photo detector 42,
and the detector 43 in the ROADM as illustrated in FIG. 16A. The
wavelength selective demultiplexer 91 can select a specified wavelength
from the WDM optical signal split by the optical splitter 62. The optical
signal of the wavelength selected by the wavelength selective
demultiplexer 91 is transmitted to a client terminal as a drop signal. In
addition, the wavelength selective demultiplexer 91 has an optical filter
function similar to the wavelength blocker 63 illustrated in FIG. 10.

[0112] At least one optical signal selected by the wavelength selective
demultiplexer 91 is guided to the low-speed photo detector 42. When a
plurality of optical singles are guided from the selective demultiplexer
91 to the low-speed photo detector 42, for example, those plurality of
optical signals may be combined by using an optical coupler or an optical
selector, for example. The low-speed photo detector 42 converts the
output light of the wavelength selective demultiplexer 91 into an
electric signal, and the detector 43 detects the path ID signal from the
electric signal.

[0113] The superimposed signal detection circuit 40 may be realized by a
wavelength selective multiplexer 92, the low-speed photo detector 42, the
detector 43 in the ROADM, as illustrated in FIG. 16B. The wavelength
selective multiplexer 92 selects one or more add signals to be inserted
into the output WDM optical signal, from a plurality of add signals.
Then, the wavelength selective multiplexer 92 combines the selected add
signals to be guided to the optical coupler 65. The wavelength selective
multiplexer 92 provides an optical filter function similar to the
wavelength blocker 63 illustrated in FIG. 10.

[0114] The output light of the wavelength selective multiplexer 92 is also
guided to the low-speed photo detector 42 using an optical splitter and
the like. The low-speed photo detector 42 converts the output light of
wavelength selective multiplexer 92 into an electric signal, and the
detector 43 detects the path ID signal from the electric signal.

[0115] In the ROADM 60 illustrated in FIG. 10, when the wavelength blocker
63 has an optical channel monitor (OCM) to control the optical filter
(the liquid crystal elements illustrated in FIG. 12), the superimposed
signal detection circuit may include the OCM instead of the low-speed
photo detector 42. In a similar manner, in the ROADM illustrated in FIG.
15, FIG. 16A, FIG. 16B, the superimposed signal detection circuit may
include the OCM provided in the wavelength selective switch 67, the
wavelength selective demultiplexer 91, and the wavelength selective
multiplexer 92 in place of the low-speed photo detector 42. According to
these configurations, since there is no need to provide a dedicated photo
detector to detect the superimposed signal, the cost may be reduced.
Furthermore, since the OCM measures the optical power for each wavelength
component of input light, it may be possible to check the detection
result of the path ID while associating with the signal wavelength. By
this function, in a case in which there was a mistake in the setting of
the optical wavelength at the transmitter for example, it becomes
possible to detect the mistake.

[0116] FIG. 17 is a flowchart illustrating a superimposed signal
transmission method of the embodiment. The processes in the flowchart are
performed by the optical transmitter illustrated in FIG. 3A or 3B, and
the superimposed signal detection circuit illustrated in FIG. 5, for
example.

[0117] In S1, the optical transmitter superimposes the path ID signal on
an optical signal carrying a data signal by frequency modulation. The
rate of the path ID signal is sufficiently low compared with the symbol
rate of the data signal. In addition, the path ID signal is realized by,
for example, a code or a tone signal.

[0118] In S2, the superimposed signal detection circuit 40 filters the
optical signal on which the path ID signal is superimposed, using the
optical filter 41. The optical filter 41 has wavelength-dependent loss.

[0119] In S3, the low-speed photo detector 42 converts the optical signal
filtered by the optical filter 41 into an electric signal. The bandwidth
of the low-speed photo detector 42 is lower than the symbol rate of the
data signal, and is higher enough to detect the path ID signal. As a
result, an electric signal waveform representing the path ID may be
obtained.

[0120] In S4, the detector 43 obtains the waveform component of an
electric signal (optical current) generated by the low-speed photo
detector 42. The waveform component corresponds to the path ID signal. In
S5, the detector 43 identifies the path ID based on the waveform
component obtained in S4. When the path ID is represented by a code, the
detector 43 identifies the path ID using a correlator. When the path ID
is represented by a tone signal, the detector 43 identifies the path ID
by frequency detection. Note that S2-S5 are an example of a superimposed
signal detection method.

[0121] S1 of the flowchart is performed by the optical transmitters 21-1
through 21-n, for example. At this time, the optical transmitters 21-1
through 21-n may superimpose the path ID signal on corresponding optical
signal at the same time. In this case, the superimposed signal detection
circuit 40 detects a plurality of path IDs at the same time, in S5. In
addition, the optical transmitters 21-1 through 21-n may superimpose the
path ID signal on a corresponding optical signal sequentially. In this
case, the superimposed signal detection circuit 40 detects each path ID
sequentially by repeating the processes in S2 through S5. By so doing,
the superimposed signal detection circuit 40 can detect the path ID
associating with wavelength λ1 through λn.

[0122] In the description above, the path ID signal that identifies the
optical path is superimposed on the optical signal carrying data signal.
However, the present embodiment is not limited to the configuration or
the method to superimpose the path ID signal on the optical signal. That
is, the present invention may be applied to a configuration and a method
to superimpose any signal on an optical signal.

[0123] In addition, the superimposed signal detection circuit and method
of the embodiment may be applied to a polarization multiplex system. In
the polarization multiplex system, each wavelength channel may carry two
optical signals (X polarization optical signal and Y polarization optical
signal) using two polarizations (X polarization and Y polarization) that
are orthogonal to each other. In this system, the optical transmitter may
superimpose different ID signals on the X polarization optical signal and
Y polarization optical signal in stead of superimposing the same ID
signal on the X polarization optical signal and Y Polarization optical
signal. Alternatively, the path ID signal may be superimposed on either
one of the X polarization optical signal and the Y polarization optical
signal.

[0124] In this case, in the superimposed signal detection circuit provided
in the optical node equipment, the polarization multiplexed optical
signal may be input to the optical filter 41 without being separated for
each polarization. The low-speed photo detector 42 converts the
polarization multiplexed optical signal filtered by the optical filter 41
into an electric signal. The detector 43 detects the path ID signal
superimposed on the X polarization optical signal and the Y polarization
optical signal, respectively. That is, as the optical filter 41 and the
low-speed photo detector 42, a device that is not dependent on the
polarization may be used. Thus, according to the superimposed signal
detection circuit and method of the embodiment, the signal superimposed
on an optical signal may be detected without using a polarization
processing device such as a polarization demultiplexer, polarization
controller and a polarizer, even in a polarization multiplex system.

[0125] All examples and conditional language recited herein are intended
for pedagogical purposes to aid the reader in understanding the invention
and the concepts contributed by the inventor to furthering the art, and
are to be construed as being without limitation to such specifically
recited examples and conditions, nor does the organization of such
examples in the specification relate to a showing of the superiority and
inferiority of the invention. Although the embodiment (s) of the present
inventions has (have) been described in detail, it should be understood
that the various changes, substitutions, and alterations could be made
hereto without departing from the spirit and scope of the invention.